1. Field of the Invention
The present invention relates generally to the field of liquid filtration using unfixed filter media, and more specifically to a method and system for removing filtered contaminants from these type filter beds during periodic cleaning cycles.
2. The Prior Art
Various types of methods and systems have been used in the prior art to remove accumulated contaminants from a bed of granular filter medium. The methods utilized to date generally have these common processing steps: 1) providing an agitation means to break-up agglomerations of filter media and contaminants typically formed during the filtration process, 2) flowing a carrier medium through the agitated granules to mobilize the contaminants away from the filter media, 3) retaining the filter media within the filter while allowing contaminants to flow out, and 4) returning the cleaned filter media back to its normal state. These four steps can be condensed to the following: 1) agitation, 2) washing, 3) separating, and 4) reforming.
With regard to common Step 1, various means are disclosed to agitate the filter media such as rotary blades and high-velocity liquid jets (See U.S. Pat. Nos. 2,521,396 and 3,992,291/3,953,333). However, both of these methods create at least two significant problems. First, rotary blade systems often have mechanical seals that require frequent maintenance. Second, the high-velocity liquid jets produce large volumes of dirty backwash water that must be stored and recycled through the process. What is needed in the art is an agitating means that does not require rotating internal baffles or impellers and minimizes liquid usage.
With regard to common step 2, the carrier medium used to flush the filter media is most commonly the clean filtrate fluid. In many systems, large volume storage of clean filtrate is required to provide surge capacity when the backwash cycle draws a high-volume rate to flush the media during this step. Some methods utilize the high-volume water jets to both agitate and back flush, which is a combination of common steps 1 and 2. However, such systems still generate large volumes of backwash liquid that must be stored and recycled back through the process. Also, it would be preferred to utilize contaminated process fluid for backwashing instead of clean filtrate. This would avoid having to have clean filtrate storage vessels and pumps specifically for periodic backwashing cycles.
With regard to common step 3, separation of the contaminants from the filter media is typically done by flowing the slurry in a continuous flow path over a cleaning element, located external to the filter housing, where interspersed larger particulates are removed from the slurry, and returning the withdrawn filter material back to the filter housing (See U.S. Pat. Nos. 3,992,291 and 3,953,333). This method adds significant cost and size to the filter since it requires various external conduits, vessels, valves and equipment. U.S. Pat. No. 4,787,987 discloses an in-situ method of separating the contaminants from the filter media by a screen, of size slightly smaller than the filter media size, contained within the vessel below the filter media. That method agitates and slurrifies the media and contaminants by action of a high-volume liquid pump. During this agitation step, make-up liquid is added to the vessel at substantially the same rate that the concentrated contaminated liquid is removed through the screen means while the filter media is retained within the filter housing.
The present invention is directed in part, to a novel means of in-situ agitation and fluidization of filter media granules to sufficiently liberate collected contaminants as rapidly as possible and flushing those contaminants from the filter housing the least amount of liquid necessary. Other advantages that are inherent in the disclosed and claimed invention will be apparent of those of ordinary skill in the art.